1. Field of the Invention
The present invention relates to an electric power steering apparatus which includes an electric motor that applies a force (steering assisting force) for assisting in a steering action made by the driver of a motor vehicle when the driver turns the steering wheel of the motor vehicle.
2. Description of the Related Art
There are known electric power steering apparatus which include an electric motor that applies a force (steering assisting force) for assisting in a steering action made by the driver of a motor vehicle in order to allow the driver to lightly turn the steering wheel of the motor vehicle {see U.S. Pat. No. 5,398,953 (hereinafter referred to as “U.S. Pat. No. 5,398,953 A”)}.
According to U.S. Pat. No. 5,398,953 A, it is disclosed that the following expression (1) is satisfied (see line 61 on column 5 through line 19 on column 6):TS≈T=TL±Tf+TI−TM  (1)where TS represents a steering torque generated on a steering wheel, T a steering torque detected by a steering torque detector (13), TL a torque generated by a load that is applied from a road surface to road wheels, Tf a torque generated by the friction of a motor, gears and other components of the road wheel mechanisms, TI a torque generated by a motor inertia, a road wheel inertia, etc., and TM a torque generated by a motor torque (see line 61 on column 5 through line 19 on column 6).
According to U.S. Pat. No. 5,398,953 A, a process shown in FIG. 2 thereof is performed on the premise of the above expression (1). Specifically, a target steering torque (Tθ) is established based on a steering angle (θ). A deviation (ΔT) between the target steering torque (Tθ) and a manual steering torque (T) is calculated. A motor current value (IT) is established depending on the deviation (ΔT). A deviation (ΔI) between the motor current value (IT) and a detected signal (23a) from a motor current detector (23) is calculated. Based on the deviation (ΔI), operations of semiconductor devices (22a through 22d) are controlled (see line 18 on column 3 through line 43 on column 4, of U.S. Pat. No. 5,398,953 A).